Several techniques are known for noninvasively determining the blood pressure of a subject. Among these is an ausculcatory method in which a cuff is wrapped around a portion of the subject's body, usually the upper arm. The cuff is inflated with air thereby pressurizing the cuff so that the cuff compresses the subject's arm and occludes the artery extending therethrough. After the artery has been occluded, the pressure in the cuff is released in a controlled fashion. During the decompression a caregiver, aided by a stethoscope, listens for sounds known as Korotkov sounds, also referred to as K-sounds. Variations in the character of the K-sounds are used to discern the time of the post-occlusion initial onset of blood flow through the artery and the subsequent re-establishment of substantially normal blood flow through the artery. The cuff pressure at the onset of blood flow and at the re-establishment of normal blood flow are taken to be the subject's systolic and diastolic blood pressure values respectively.
A similar ausculcatory method relies on a microphone in the cuff rather than a caregiver listening for the K-sounds. An algorithm executed by a processor identifies the K-sounds and the associated systolic and diastiolic pressures.
Another method is an oscillometric method. The oscillometric method is similar to the microphone assisted ausculcatory method except that instead of using a microphone the cuff includes a pressure transducer, and instead of relying on the K-sounds as described above, the oscillometric technique relies on oscillations in the subject's blood pressure, that create relatively small disturbances in the cuff pressure, to identify the subject's systolic and diastolic blood pressure. The oscillations appear during the inflation phase of the oscillometric method, increase in amplitude to a peak amplitude with increasing cuff pressure, and then decrease in amplitude and vanish with further increases in cuff pressure. Similar oscillations appear during the deflation phase of the oscillometric method. The oscillations increase in amplitude to a peak amplitude with decreasing cuff pressure, and finally vanish with further decreases in cuff pressure. The amplitude of the oscillations is alternatively referred to as pulse height. The peak amplitude or pulse height is typically about 1 to 3 mm Hg. A graph of pulse height vs. cuff pressure is referred to as the pulse envelope. The pulse envelope has an ascending side corresponding to the pulses of increasing amplitude and a descending side corresponding to the pulses of decreasing amplitude. A processor analyzes the pulse envelope and takes the cuff pressure corresponding to the peak pulse height as an estimate of the subject's mean arterial pressure (MAP). The processor equates the subject's systolic blood pressure to the cuff pressure on the ascending side of the pulse envelope at a predefined fraction (less than 1.0) of the peak pulse amplitude. The processor equates the subject's diastolic blood pressure to the cuff pressure on the descending side of the pulse envelope at a predetermined fraction (also less than 1.0) of the peak pulse amplitude. The predefined and predetermined fractions may or may not be equal to each other.
Blood pressure may also be determined by way of cuffless techniques. One cuffless technique measures pulse transit time (PTT). PTT is the time it takes a pulse wave to propagate from a more upstream location to a more downstream location in a subject's artery. PTT is considered to be a good indication of MAP. A processor executes an algorithm to determine systolic and diastolic pressure from the MAP.
Although the foregoing techniques are widely used, they either do not account for the effects of arterial compliance on the blood pressure determination, or do not do so satisfactorily. Accordingly, the subject matter described herein discloses a system for correcting a blood pressure value to account for arterial compliance, a system for determining a blood pressure including a correction for arterial compliance, and a method of determining the correction.